Data Structures and Algorithms
with Object-Oriented Design Patterns in Java 
Fragmentation is a phenomenon that occurs in a long-running program that has undergone garbage collection several times. The problem is that objects tend to become spread out in the heap. Live objects end up being separated by many, small unused memory regions. The problem in this situation is that it may become impossible to allocate memory for an object. While there may indeed be sufficient unused memory, the unused memory is not contiguous. Since objects typically occupy consecutive memory locations it is impossible to allocate storage.
The mark-and-sweep algorithm does not address fragmentation. Even after reclaiming the storage from all garbage objects, the heap may still be too fragmented to allocate the required amount of space. The next section presents an alternative to the mark-and-sweep algorithm that also defragments (or compacts ) the heap.
Copyright © 1998 by Bruno R. Preiss, P.Eng. All rights reserved.